1. Field of the Invention
The present invention relates to a centering device for blanking dies, particularly a centering device to be associated with the rotary die of a mold for producing laminations for electric motors.
2. Discussion of Related Art
The stators and rotors of several types of electric motors are made by packing of a plurality of suitably shaped laminations, which are made of ferromagnetic material. The individual laminations are generally obtained from metal laminates that are subjected to molding and punching processes. The thus-obtained laminations are coupled to each other; in particular, they are stacked to form the core of a rotor or to form a stator. Each lamination is provided with slots, which along with the slots of the other laminations, define the slots for housing the stator/rotor windings or for housing the melt material alternatively used (generally die-cast aluminum).
The laminations used for making rotors of electric motors can be coupled such that the rotor has straight or skew slots, such as having a helical development. In other words, the laminations can be stacked on each other without offset, such that the slots for the windings are overlapped to form a straight slot, or with an angular offset, such that the slots of a first lamination result to be rotated relative to the matching slots of a second lamination adjacent thereto, in order to form a slot for the winding which is either skew or helical.
The laminations are coupled to form a pack having the desired height, corresponding to the height of the rotor or stator of the electric motor to be made. Regardless of the slot shape, when the pack is made up of a large number of laminations, any difference in the thickness that can be found between the different portions of the laminations can lead to inaccurate assembly.
For those packs made up of a large number of laminations, for example more than 100, a “compensation” may be required during the manufacturing step. The compensation is carried out by stacking the laminations such that the pack mass is evenly distributed relative to the axis thereof. For example, the rotors or stators are “compensated” by packing each lamination such as to be offset by a preset angle, such as 900 relative to the adjoining lamination (and this can be provided for all the laminations in the pack or laminations sets) such that any non-uniformity of an individual lamination is evenly distributed relative to the axis of rotation of the lamination pack. The compensation of the lamination pack can be required both when manufacturing stator packs and when manufacturing rotor packs.
Generally, the lamination coupling is carried out by providing each lamination with one or more bosses. Usually, the laminations are stacked during the manufacturing step, directly within the mold and during the punching step, by forcing the bosses of a lamination in the matching bosses of the adjoining lamination in the same stack.
The molds are provided with a die, which by cooperating with a punch, provides to cut the metal laminate being fed to the mold, thereby separating the laminations. The punch is fastened to a mold portion, which moves in a vertical and reciprocating manner on the laminate, which remains interposed between the punch and the die. The laminate feeding movement is coordinated with the punch movement, such that—upon each downward movement of the punch—new portions of the laminate are intercepted by the punch and die to be cut.
When the “compensation” is required for the lamination pack, the mold is equipped with a die to be rotated about its own axis. The rotary die provides to make the individual laminations (by cutting the laminate in cooperation with the punch) such as to be offset relative to the previously worked lamination, such as to compensate any non-uniformity in the mass distribution of the lamination pack to be made.
The operation of the mold provides that the punch and die carry out the cutting of a first lamination. In a later time, after the punch has been raised from the laminate and the latter has been fed forward, the die rotates about its own axis according to a preset angle. The punch is forced once again on the laminate to carry out the cutting of a second lamination. The second lamination is angularly offset relative to the previously-cut lamination. The offset angle corresponds to the angle of rotation of the die.
The die is fixed to a support sleeve that is pivotally fastened to the lower portion of the mold. The sleeve is fitted within a seat of the mold and is, in turn, supported by bearings. A suitable motor rotates the sleeve, and thus also the die, according to the desired angle.
Traditionally, the portion of the mold supporting the die is the stationary, lower portion, while the portion of the mold supporting the punch is the upper portion, which is vertically moved in a reciprocating manner. The upper portion of the mold is suitably guided during the vertical movement thereof, such that the punch and die are always properly aligned with each other.
The guide device of the upper portion of the mold comprises at least one pilot “column”, which is generally a rigid rod fastened to the upper portion of the mold that slidably engages the support sleeve of the punch die and engages a centering bush, which is also fastened to the sleeve. When the punch is moved down to the laminate to carry out the cutting, the pilot column also vertically moves, thereby bringing a distal end thereof in engagement with the centering bush, opposite the punch. Thereby, the guide device holds the punch and die centered during the cutting step.
Current molds can provide high operating speeds. For example, the punch and pilot column can be operated 300 times/minute. The accuracy of the guide device in centering the two portions of the mold (upper and lower) and thus in centering the punch relative to the die, is important to ensure high quality and output standards.
Disadvantageously, the traditional guide devices do not allow a fine alignment to be achieved for the die relative to the desired position, and consequently relative to the punch, when the die is rotated. The slidable coupling between the pilot column and the centering bush provides that a clearance, though minimal, is left between these elements. In other words, the section area of the distal end of the pilot column must be lower than the section area of the seat of the centering bush in which it is fitted. Thereby, any damaging interference is avoided between the pilot column and the bush, which may cause jamming.
The clearance that must be provided between the pilot column and the centering bush is a restraint for the proper and repetitive positioning of the die during the laminate processing, which means that the maximum precision that can be obtained by means of the centering device is equal to the clearance between the column and bush. In the current practice, undesired misalignments between the packed laminations are mostly caused by the non-repetitiveness of the die positionings. In other words, the die rotates prior to a new cutting action, but due to the clearance provided for the elements of the guide device, the re-positionings are not identical over time, with clear negative consequences on the process accuracy.
A further drawback with the traditional molds is that the motor that rotates the sleeve within the seat thereof is generally subjected to a systematic, though minimum, error, which determines slight inaccuracies when the sleeve is angularly positioned. Consequently, the die can result improperly aligned relative to the punch. After a number of cutting cycles, these inaccuracies are likely to result in localized wear of the centering bush, i.e. several points on the centering bush are worn before others.
Disadvantageously, in the traditional molds, the centering bush is subjected to abrasion caused by the metal dust obtained from the cutting of the laminations, which dust deposits on the bush and on the distal end of the pilot column engaging the same.
Among mold manufacturers, the need has been felt for some time to maximize the accuracy of positioning of the rotary cutting die.
One aspect of the present invention is to provide a centering device for rotary cutting dies which solves the drawbacks of traditional devices in a simple and effective manner, thus allowing a high repeatability of the positionings of the relative die to be obtained.
A further aspect of the present invention to provide a centering device for rotary cutting dies, which allows minimizing, during the manufacturing step, the inaccuracies in the alignment of the laminations in a same pack.
Another aspect of the present invention to provide a centering device for rotary cutting dies which provides recovering and canceling the clearances relative to the positioning of the relative die.